9,160 research outputs found
Frequency and temporal effects in linear optical quantum computing
Typically linear optical quantum computing (LOQC) models assume that all
input photons are completely indistinguishable. In practice there will
inevitably be non-idealities associated with the photons and the experimental
setup which will introduce a degree of distinguishability between photons. We
consider a non-deterministic optical controlled-NOT gate, a fundamental LOQC
gate, and examine the effect of temporal and spectral distinguishability on its
operation. We also consider the effect of utilizing non-ideal photon counters,
which have finite bandwidth and time response.Comment: 10 pages, 9 figures, replaced with published versio
Modeling photo-detectors in quantum optics
Photo-detection plays a fundamental role in experimental quantum optics and
is of particular importance in the emerging field of linear optics quantum
computing. Present theoretical treatment of photo-detectors is highly idealized
and fails to consider many important physical effects. We present a physically
motivated model for photo-detectors which accommodates for the effects of
finite resolution, bandwidth and efficiency, as well as dark-counts and
dead-time. We apply our model to two simple well known applications, which
illustrates the significance of these characteristics.Comment: 8 pages, 7 figure
Quantum Metrology in the Kerr Metric
A surprising feature of the Kerr metric is the anisotropy of the speed of
light. The angular momentum of a rotating massive object causes co- and
counter-propagating light paths to move at faster and slower velocities,
respectively as determined by a far-away clock. Based on this effect we derive
ultimate quantum limits for the measurement of the Kerr rotation parameter
using a interferometric set up. As a possible implementation, we propose a
Mach-Zehnder interferometer to measure the "one-way height differential" time
effect. We isolate the effect by calibrating to a dark port and rotating the
interferometer such that only the direction dependent Kerr-metric induced phase
term remains. We transform to the Zero Angular Momentum Observer (ZAMO) flat
metric where the observer see . We use this metric and the Lorentz
transformations to calculate the same Kerr phase shift. We then consider
non-stationary observers moving with the planet's rotation, and find a method
for cancelling the additional phase from the classical relative motion, thus
leaving only the curvature induced phase.Comment: 9 pages, 7 figures, closest to published versio
Estimating space-time parameters with a quantum probe in a lossy environment
We study the problem of estimating the Schwarzschild radius of a massive body
using Gaussian quantum probe states. Previous calculations assumed that the
probe state remained pure after propagating a large distance. In a realistic
scenario, there would be inevitable losses. Here we introduce a practical
approach to calculate the Quantum Fisher Informations (QFIs) for a quantum
probe that has passed through a lossy channel. Whilst for many situations loss
means coherent states are optimal, we identify certain situations for which
squeezed states have an advantage. We also study the effect of the frequency
profile of the wavepacket propagating from Alice to Bob. There exists an
optimal operating point for a chosen mode profile. In particular, employing a
smooth rectangular frequency profile significantly improves the error bound on
the Schwarzschild radius compared to a Gaussian frequency profile.Comment: 14 pages, 18 figure
Improving the Fidelity of Optical Zeno Gates via Distillation
We have modelled the Zeno effect Control-Sign gate of Franson et al (PRA 70,
062302, 2004) and shown that high two-photon to one-photon absorption ratios,
, are needed for high fidelity free standing operation. Hence we
instead employ this gate for cluster state fusion, where the requirement for
is less restrictive. With the help of partially offline one-photon and
two-photon distillations, we can achieve a fusion gate with unity fidelity but
non-unit probability of success. We conclude that for , the Zeno
fusion gate will out perform the equivalent linear optics gate.Comment: 6 pages, 11 figures, Submitted to PR
Error models for mode-mismatch in linear optics quantum computing
One of the most significant challenges facing the development of linear
optics quantum computing (LOQC) is mode-mismatch, whereby photon
distinguishability is introduced within circuits, undermining quantum
interference effects. We examine the effects of mode-mismatch on the parity (or
fusion) gate, the fundamental building block in several recent LOQC schemes. We
derive simple error models for the effects of mode-mismatch on its operation,
and relate these error models to current fault tolerant threshold estimates.Comment: 6 pages, 7 figure
Quantum Correlations in Nonlocal BosonSampling
Determination of the quantum nature of correlations between two spatially
separated systems plays a crucial role in quantum information science. Of
particular interest is the questions of if and how these correlations enable
quantum information protocols to be more powerful. Here, we report on a
distributed quantum computation protocol in which the input and output quantum
states are considered to be classically correlated in quantum informatics.
Nevertheless, we show that the correlations between the outcomes of the
measurements on the output state cannot be efficiently simulated using
classical algorithms. Crucially, at the same time, local measurement outcomes
can be efficiently simulated on classical computers. We show that the only
known classicality criterion violated by the input and output states in our
protocol is the one used in quantum optics, namely, phase-space
nonclassicality. As a result, we argue that the global phase-space
nonclassicality inherent within the output state of our protocol represents
true quantum correlations.Comment: 5 pages, 1 figure, comments are very welcome
Optimal cloning for finite distributions of coherent states
We derive optimal cloning limits for finite Gaussian distributions of
coherent states, and describe techniques for achieving them. We discuss the
relation of these limits to state estimation and the no-cloning limit in
teleportation. A qualitatively different cloning limit is derived for a
single-quadrature Gaussian quantum cloner.Comment: 15 pages RevTeX, 6 figures eps, submitted to PR
Non-deterministic approximation of photon number discriminating detectors using non-discriminating detectors
We present a scheme for non-deterministically approximating photon number
resolving detectors using non-discriminating detectors. The model is simple in
construction and employs very few physical resources. Despite its
non-determinism, the proposal may nonetheless be suitable for use in some
quantum optics experiments in which non-determinism can be tolerated. We
analyze the detection scheme in the context of an optical implementation of the
controlled-NOT gate, an inherently non-deterministic device. This allows the
gate's success probability to be traded away for improved gate fidelity,
assuming high efficiency detectors. The scheme is compared to two other
proposals, both deterministic, for approximating discriminating detectors using
non-discriminating detectors: the cascade and time division multiplexing
schemes.Comment: 5 pages, 7 figures (published version
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